1. Field of the Invention
The present invention generally relates to a flat panel display and, more particularly, to a flat panel display having a high efficiency in which a cathode electrode has a different thickness for each of red (R), green (G), and blue (B) pixels, and a method of fabricating the same.
2. Description of the Related Art
A conventional active matrix organic light emitting device includes an anode electrode which is connected to a thin film transistor, a cathode electrode, and a red (R), green (G), or blue (B) organic thin film layer formed therebetween. The organic thin film layer can include multiple layers. Examples of such layers include a hole injecting layer, a hole transporting layer, R, G, B organic emission layers, a hole blocking layer, an electron transporting layer, and an electron injecting layer.
The cathode electrode typically uses a metal electrode, which may be formed of metals such as aluminum (Al), or metals alloys such as Magnesium (Ma)-Silver (Ag). These and other metals and metal alloys facilitate electron transportation and at the same time, protect the underlying organic thin film layer. To render the display more stable and less susceptible to electromagnetic interference, a two-layer cathode electrode is typically used. However it is nearly impossible to obtain optimized efficiency and color coordinates because the conventional cathode electrode is typically formed in a uniform thickness for each of the R, G, or B pixels.
FIG. 1 shows a cross-sectional view of a conventional active matrix organic light emitting device which includes a conventional two-layer structured cathode electrode.
Thin film transistors for R, G, B unit pixels 101, 103, and 105, respectively, are formed on a buffer layer 110 of a substrate 100. The thin film transistors include semiconductor layers 120, 130 and 140 respectively having source/drain regions 121 and 125, 131 and 135, and 141 and 145, gates 161, 163, and 165 formed on a gate insulating layer 150, and source/drain electrodes 181 and 185, 191 and 195, and 201 and 205 formed on an insulating interlayer layer 170.
Anode electrodes, 220, 230 and 240, which are lower electrodes for the R, G, B unit pixels 101, 103, and 105, are formed on a passivation layer 210 and are connected to one of the drain electrodes 185, 195, and 205, respectively, through via holes 107, 109, 111.
Further, a pixel defining layer 250 for isolating respective R, G, B unit pixels 101, 103, 105 is formed on the passivation layer 210. R, G, B organic thin film layers 271, 273, 275, respectively, are formed on the anode electrodes 220, 230, 240 for the R, G, B unit pixels 101, 103, 105, respectively, exposed through openings 261, 263, 265 of the pixel defining layer 250. A cathode electrode 280 is formed as an upper electrode on an entire surface of the substrate 100.
The anode electrodes 220, 230, 240 include first anode electrodes 221, 231, 241, respectively, each having high reflectivity, and second anode electrodes 225, 235, 245 for adjusting a work function. The anode electrodes 220, 230, 240 have equal thicknesses for each of the R, G, B unit pixels 101, 103, 105, respectively.
The cathode electrode 280 is formed of a first cathode electrode 281 constructed of a metal or metal alloy and a second cathode electrode 285 constructed of a transparent conductive layer having excellent stability, and is formed on the entire surface of the substrate 100 with a uniform thickness. Exemplary metals and metal alloys commonly used include Lithium Fluoride (LiF) or Magnesium and Silver alloys (Mg:Ag). Exemplary transparent conductive materials include Indium tin oxide (ITO) and Indium zinc oxide (IZO).